Wireless communication with a medical implant

ABSTRACT

An apparatus for providing transdermal wireless communication includes medical implant circuitry; a transceiver coupled to the medical implant circuitry; a first metal surface having an end portion and a base portion; a second metal surface parallel to the first metal surface and connected to the first metal surface by a conductor, the second metal surface being separated from the first metal surface by a dielectric layer; a first radiating element tuned to a first frequency and disposed within the dielectric layer between the first metal surface and second metal surface; and a feed structure in electrical communication with the transceiver and the first radiating strip. The first radiating element has a first reactive portion at a first end thereof, a second reactive portion at a second end thereof, and a first radiating strip extending between the first reactive portion and the second reactive portion.

TECHNICAL FIELD

This invention relates to medical implants, and more particularly tocommunication with a medical implant.

BACKGROUND

Among the known medical implants are those that either receiveinformation from a transmitter outside the body or transmit informationto a receiver located outside the body. Such communication is mostconveniently carried out by causing electromagnetic waves to propagatebetween an intra-corporal medical implant and an extra-corporal basestation.

A difficulty with the use of electromagnetic waves arises from theirtendency to be attenuated when traveling within the human body. Althoughattenuation decreases with increasing wavelengths, the use of longerwavelengths typically requires the use of large antennas.

In 1999, the United States Federal Communication Commission (“FCC”)allocated the Medical Implant Communication Service (“MICS”) band, whichextends between 402 MHz and 405 MHz, as available for use by medicalimplants. Although the MICS band represents an attempt at compromise, itis still the case that body tissues significantly attenuateelectromagnetic waves propagating at MICS frequencies. As a result, thedistance between the base station and the implant must be small. Infact, in many applications, the base station's receiving antenna isplaced on or within inches of the skin.

The limited range of known medical implant communication systems posesfew problems when one wishes to establish communication with an implantinfrequently. For example, if one only needed to communicate with animplant during a monthly clinical appointment, it would not beinconvenient to have to hold a receiver next to the skin for shortperiods.

However, in some applications, one would like to communicateperiodically or intermittently with an implant over an extended period.For example, one might need to monitor a measured value at frequenttimes or may need to cause an implant to release a drug at certain timesor in response to certain conditions.

Under the foregoing conditions, it would be convenient to establishcommunication between an implant and a base station within the same roomas a patient, but in some unknown and changing direction and distancerelative to the patient.

In principle, one could extend the communication range of an implant bytransmitting with more power. One difficulty that arises, however, isthat the FCC imposes a limit on the amount of power that can betransmitted. Another difficulty that arises is that the implant's powersupply is finite, and high power transmission is apt to drain it morequickly.

An exemplary telemetry apparatus for an implantable medical device isthat described in U.S. Pat. No. 6,574,203 (Von Arx).

Antennas for implantable medical devices are disclosed in U.S. Pat. No.6,809,701 (Amundson et al.), U.S. Pat. No. 7,149,578 (Edvardsson), U.S.Pat. No. 5,861,019 (Sun et al.), and U.S. Patent Publication2005/0154428 (Bruinsma).

SUMMARY

The invention is based on the recognition that a non-omnidirectionalantenna on a medical implant will interact with the patient's body insuch a way as to yield a nearly omnidirectional radiation pattern.

In one aspect, the invention features an apparatus for providingtransdermal wireless communication. The apparatus includes medicalimplant circuitry, a transceiver coupled to the medical implantcircuitry, a first metal surface having an end portion and a baseportion, a second metal surface parallel to the first metal surface andconnected to the first metal surface by a conductor, and separated fromthe first metal surface by a dielectric layer, a first radiating elementtuned to a first frequency and disposed within the dielectric layerbetween the first metal surface and second metal surface. The firstradiating element has a first reactive portion at a first end thereof, asecond reactive portion at a second end thereof, and a first radiatingstrip extending between the first reactive portion and the secondreactive portion. The apparatus further includes a feed structure inelectrical communication with the transceiver and the first radiatingstrip.

In some embodiments, the first reactive portion includes a firstcapacitive structure, and the second reactive portion includes a secondcapacitive structure.

In other embodiments, the first reactive portion includes a firstconductive planar portion having a dimension in excess of a width of thefirst radiating strip, and the second reactive portion includes a secondconductive planar portion having a dimension in excess of the width ofthe first radiating strip.

Alternative embodiments include those in which the first reactiveportion includes an inductive structure, and those in which the firstreactive portion includes a first conducting strip disposed to follow afirst serpentine path, and the second reactive portion includes a secondconducting strip disposed to follow a second serpentine path, as well asthose in which the first reactive portion of the first radiating elementincludes an inductive structure and the second reactive portion includesa capacitive structure.

Yet other embodiments include those in which the feed structure isseparated from the first radiating strip by a dielectric, and those inwhich the feed structure is capacitively coupled to the first radiatingstrip.

Other embodiments include a second radiating element tuned to a secondfrequency and disposed between the first reactive portion of the firstradiating element and the second reactive portion of the first radiatingelement. In some such embodiments, the feed structure can provide asignal of the first frequency and a signal of the second frequency toboth the first radiating element and the second radiating element. Inother such embodiments, the feed structure is capacitively coupled toboth the first radiating element and the second radiating element. Inyet other such embodiments, the second radiating element is tuned to afrequency between approximately 2 GHz and 2.5 GHz.

Yet other embodiments include those in which a planar surface forms thesecond metal surface. In some such embodiments, the planar surfaceincludes a surface of a housing.

Other embodiments include those in which the second metal surfaceincludes a planar surface of a housing.

In another embodiment, the end portion of the first metal surface isdisposed over the first reactive portion of the first radiating elementand the base portion of the first metal surface is disposed over thesecond reactive portion of the first radiating element.

Other embodiments include those in which the first radiating elementdefines a region at which an electric field supported by the firstradiating element has its maximum amplitude, and in which at least oneof the end portion and base portion is disposed over the defined region,those in which at least one of the end portion and base portion isdisposed over a region at which a gradient vector of electric fieldamplitude reverses direction, those in which at least one of the endportion and base portion is disposed to intercept electric field linesin a region at which an electric field supported by the first radiatingelement reaches its maximum amplitude, and those in which at least oneof the end portion and base portion is disposed to block a selectedportion of an electric field supported by the first radiating element,with examples of the selected portion of the electric field including aportion having an amplitude in excess of a threshold, and a portionhaving a gradient vector that reverses direction.

Additional embodiments include those in which the first radiatingelement is tuned to a first frequency within the MICS band, and those inwhich the first radiating element is tuned to a first frequency between400 MHz and 405 MHz.

At least one embodiment of the apparatus further includes a neckextending between the end portion and the base portion.

Other embodiments of the apparatus include those in which at least oneof the first metal surface and the second metal surface includes aplanar surface, those in which the first and second metal surfacesinclude grounded surfaces, and those in which the first and second metalsurfaces include ground planes.

In another aspect, the invention features an apparatus for providingtransdermal wireless communication in a selected direction, theapparatus including medical implant circuitry; a transceiver coupled tothe medical implant circuitry; and a first metal surface disposed in aplane perpendicular to the selected direction. The first metal surfacehas an end portion, and a base portion. The apparatus further includes afirst planar radiating element tuned to a first frequency and disposedon a dielectric layer above the first metal surface, the first planarradiating element having a first reactive portion at a first endthereof, a second reactive portion at a second end thereof, and a firstplanar radiating strip extending between the first reactive portion andthe second reactive portion; and a feed structure in electricalcommunication with the transceiver and the first planar radiating stripfor providing the carrier signal to the first planar radiating strip.

In some embodiments, the apparatus further includes a neck extendingbetween the end portion and the base portion.

In yet another aspect, the invention features an apparatus for providingwireless communication across the skin of a patient, the apparatusincluding: medical implant circuitry; a transceiver coupled to themedical implant circuitry; a feed configured to receive a signal fromthe transceiver; a planar radiating element coupled to the feed; and afield stop disposed to block radiation from selected portions of anelectric field distribution supported by the planar radiating element.

Another aspect of the invention is a method for providing transdermalcommunication, the method including causing a current on an antennaimplanted inside a patient, the antenna supporting an electromagneticfield having a near-field component and a far-field component; shieldingthe near-field component, thereby trapping energy contained in thenear-field component and reducing the extent to which the energy in thenear-field component interacts with the patient; and allowing thepropagation of the far-field component through the skin of the patient.

Practices of the method include those in which shielding the near-fieldcomponent includes placing a conductive plane between a reactive portionof the antenna and the skin, those in which shielding the near-fieldcomponent includes placing a conductive plane over a first end of theantenna and a second end of the antenna, and those that further includeselecting the antenna to be a radiating strip.

Another aspect of the invention is a method of providing wirelesscommunication between a medical implant and a base station across theskin of a patient in the presence of a mismatch between the permittivityof the patient's skin layer and the permittivity of a medium surroundingthe patient. Such a method includes communicating with a transceiver ofa medical implant that has been implanted under the skin of a patient;causing an antenna on the medical implant to launch an electromagneticwave carrying energy, the energy having a first portion traveling in afirst direction and a second portion traveling in a direction other thanthe first direction, the first and second portions having differentmagnitudes, wherein a portion of the first portion enters a layer of thepatient and causes an endoperipheral wave that propagates within theperipheral layer, and wherein as the endoperipheral wave propagateswithin the peripheral layer, a portion of the energy carried by theendoperipheral wave exits the endoperipheral layer and enters asurrounding medium, the ratio of the portion of the energy that exitsthe skin layer being dependent on the extent of the mismatch between thepermittivity of the endoperipheral layer and the permittivity of thesurrounding medium.

Yet another aspect of the invention features a method of providingwireless communication between a medical implant in a patient and a basestation. Such a method includes causing an antenna on the medicalimplant to launch a wave having a first portion in a first direction anda second portion in a second direction, the first and second portionshaving differing magnitudes, wherein a portion of the first portionenters a biological waveguide defined by a constituent of the body ofthe patient, the biological waveguide having a first permittivity thatdiffers from the permittivity of the medium surrounding the basestation; whereby the wave launched into the biological waveguide becomesa guided wave having an energy, and wherein as the guided wavepropagates in the biological waveguide, a portion of the energy escapesthe biological waveguide and enters the medium surrounding the basestation; and wherein the ratio of energy escaping the biologicalwaveguide to the energy remaining in the biological waveguide depends onthe ratio between the permittivity of the biological waveguide and thepermittivity of the medium surrounding the base station.

In one practice, the biological waveguide includes a portion of theskin.

Another aspect of the invention features a method of determining apreferred patient orientation for establishing communication between amedical implant inside a patient and a base station outside the patient.Such a method includes, following the healing of an incision caused byimplantation of a medical implant inside a patient, determining an anglebetween an implant axis of the implant and a patient axis of thepatient; on the basis of the angle, determining an optimal orientationof the patient relative to the base station for establishing wirelesscommunication between the medical implant and the base station; andproviding, to the patient, information representative of the optimalorientation.

In another aspect, the invention features an apparatus for providingenergy to first and second antennas. The apparatus includes a firstsection of a microstrip transmission line, the first section extendingfrom a feedpoint along an axis; a first load for coupling to the firstantenna, the first load being connected to a distal end of the firstsection; a second section of microstrip transmission line, the secondsection extending along the axis and having a proximal end connected tothe first pair of microstrip transmission line stubs; and a second loadconnected to a distal end of the second section for coupling to thesecond antenna; wherein the lengths of the first and second sections areselected to cause an electromagnetic wave having a first frequency toencounter an impedance mismatch at the first load and an impedance matchat the second load, and to cause an electromagnetic wave having a secondfrequency to encounter an impedance mismatch at the second load and animpedance match at the first load.

In another aspect, the invention features an apparatus for providingenergy to first and second antennas. The apparatus includes a firstsection of a microstrip transmission line, the first section extendingfrom a feedpoint along an axis; a first load for coupling to the firstantenna, the first load being connected to a distal end of the firstsection; a second section of microstrip transmission line extending fromthe feedpoint and along a direction parallel to and offset from theaxis; and a second load for coupling to the second antenna, the secondsection being connected to a distal end of the second section; whereinthe lengths of the first and second sections are selected to cause anelectromagnetic wave having a first frequency to encounter an impedancemismatch at the first load and an impedance match at the second load,and to cause an electromagnetic wave having a second frequency toencounter an impedance mismatch at the second load and an impedancematch at the first load.

In one embodiment, the apparatus includes a third section of microstriptransmission line extending from the feedpoint and along a directionparallel to and offset from the axis and offset from the second section,and a third load for coupling to the second antenna, the third sectionbeing connected to a distal end of the third section; wherein thelengths of the first, second, and third sections are selected to causean electromagnetic wave having a first frequency to encounter animpedance mismatch at the first load and an impedance match at thesecond and third loads, and to cause an electromagnetic wave having asecond frequency to encounter an impedance match at the first load andan impedance mismatch at the second and third loads.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription, the claims, and the drawings, in which:

DESCRIPTION OF DRAWINGS

FIG. 1 shows a medical implant in communication with a base station;

FIG. 2 shows the medical implant of FIG. 1 in more detail;

FIG. 3 is a block diagram of the wireless communication system of themedical implant of FIG. 2;

FIG. 4 is a flow chart of a communication protocol carried out by thebase station of FIG. 1;

FIG. 5 is a flow chart of a communication protocol carried out by thewireless communication system of FIG. 3;

FIG. 6 shows the dissipation of energy associated with a conventionalomnidirectional antenna;

FIG. 7 shows the propagation of endodermal waves associated with theantenna associated with the wireless communication system of FIG. 3;

FIG. 8 is a transverse cross section of the medical implant shown inFIG. 2;

FIG. 9 is an exploded isometric view of the antenna system shown in FIG.8;

FIG. 10 is a detailed view of the radiating archipelago of the antennashown in FIG. 8;

FIG. 11 is a detailed view of the top ground plane of the antenna shownin FIG. 8;

FIG. 12 is an alternative to the radiating archipelago shown in FIG. 10,in which the reactive portions provide an inductance rather than acapacitance;

FIG. 13 shows an alternative to the radiating archipelago of FIG. 10;

FIG. 14 shows details of the feed structure shown in FIG. 9;

FIGS. 15A and 15B show matching circuits from FIG. 3;

FIG. 16 shows an antenna feed thru for feeding the antenna of FIG. 9;

FIG. 17 shows a process for forming a dielectric layer on the antennasystem shown in FIG. 8;

FIG. 18 shows a structure for providing capacitive coupling between anantenna and a feed;

FIG. 19 shows an alternate embodiment of the radiating archipelago ofFIG. 13;

FIGS. 20 and 21 show additional embodiments of a feed structure for theantenna system shown in FIG. 9;

FIGS. 22 and 23 show representative three-dimensional patterns for theantenna system of FIG. 8;

FIGS. 24 and 25 show representative slices through the three-dimensionalpatterns shown in FIGS. 22 and 23; and

FIGS. 26 and 27 show antenna gain for antennas that have been implantedin a piece of meat.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1 shows a medical implant 10, sometimes referred to as an“implantable medical device,” in a patient 12. The medical implant 10 isone that either performs actions in response to instructions, transmitsdata, or both. For example, the medical implant 10 could be one thatreleases drugs in response to a stimulus. An example of an implant forcontrolled release or exposure of the contents of an implanted reservoiris described in U.S. Patent Pub. 2004/0121486 (Uhland et al.), entitled“Controlled Release Device and Method Using Electrothermal Ablation,”the contents of which are herein incorporated by reference. The medicalimplant 10 could be one that performs physiological measurements, suchas measuring glucose levels, cardiac signals, or blood pressure levels.One implant for measuring glucose is that disclosed in U.S. Patent Pub.2005/0096587 (Santini), entitled “Medical Device for Sensing Glucose,”the contents of which are incorporated herein by reference.

As used herein, the term “medical implant” refers to active implantablemedical devices. An “active implantable medical device” is a medicaldevice that uses electricity or other energy, and is partly or totallyinserted into a human or animal body or a natural orifice by means of asurgical or medical procedure, and is typically expected to remain therefor several days, weeks, months, or years after the procedure iscompleted. The term “medical device” refers to a manufactured productthat is used to prevent, diagnose, treat, or monitor human or animaldisease or injuries, or to investigate, replace, modify, or maintainanatomical structures or physiological functions. Manufactured productsthat achieve results by pharmacological, immunological, or metabolicmeans are not medical devices. However, the results achieved by medicaldevices may be assisted by these means. Representative examples ofmedical implants suitable for use in/with the present antenna devicesand telemetry methods include pacemakers, cardioverter-defibrillators,nerve and muscle stimulators, deep brain stimulators, drug deliverydevices (e.g., drug pumps), cardiomyostimulators, cochlear implants,artificial organs (e.g., artificial hearts), biological sensors, andcardiac and other physiologic monitors. The medical implant may provideof a combination of these functionalities. In one embodiment, themedical implant comprises a multi-reservoir containment device for thecontrolled in vivo exposure or release of reservoir contents, asdescribed for example in U.S. Pat. No. 6,527,762 (Santini et al.), U.S.Pat. No. 6,491,666 (Santini et al.), U.S. Pat. No. 6,551,838 (Santini etal.), U.S. Pat. No. 7,226,442 (Sheppard et al.), U.S. Patent ApplicationPublication 2004/0121486 (Uhland et al.), U.S. Patent ApplicationPublication 2005/0096587 (Santini et al.), U.S. Patent ApplicationPublication 2005/0267440 (Herman et al.), and U.S. Patent ApplicationPublication 2008/0015494 (Santini et al.), the contents of which are allincorporated herein by reference.

It is generally useful to provide such medical implants 10 with awireless link to a base station 14 located near the patient 12. As amatter of convenience, it is useful for the wireless link to be suchthat the patient 12 may stray a limited distance from the base station14 without interrupting communication. This would enable the wirelesslink to be used unobtrusively. For example, if the range of the wirelesslink is on the order of the size of a typical household room, such as abedroom, or a typical hospital room, and if radiation exits the patient12 omnidirectionally, it is possible for the patient 12 to be anywherewithin the room without disrupting wireless communication between theimplant 10 and the base station 14.

As used herein, terms such as “omnidirectional” and “omnidirectionally”are used to describe receiving or sending radio waves equally well inall directions in a principal plane of an antenna. The term “equallywell” is not intended to imply strict and unvarying equality but isintended to encompass minor deviations from equality.

FIG. 2 shows the medical implant 10 in more detail. The medical implant10 features a generally elliptical housing 15 having a major axis 16.The housing 15 is typically a biocompatible metal, such as titanium ortitanium alloy shell, with the metal forming the shell having a wallthickness of about 0.3 mm. In some embodiments, the overall thickness ofthe housing 15 is 8.2 mm. In other embodiments, the overall thickness ofthe housing 15 is 10-11 mm. An elliptical locking ring 18 having similartransverse dimensions as the housing 15 holds an RF transparent cover ordielectric shield 20 in place above an antenna structure (not shown)connected to a transceiver (not shown). The cover 20 functions toprotect the antenna structure from contact with bodily fluids and/ortissues. The locking ring 18, like the housing 15, is typically abiocompatible metal, such as titanium. A suitable material for anRF-transparent cover 20 or dielectric shield is non-conducting material,such as polyethylene, having a thickness of about 0.4 mm. Alternately,one could fill the space with a biocompatible epoxy, which would then bethe cover 20.

When implanted, orientation of the major axis 16 in a direction parallelto the patient's spine results in an omnidirectional pattern in a planetransverse to the patient's spine. This configuration is thus preferablefor signal transmission. However, it may be more comfortable for thepatient 12 if the surgeon were to orient the major axis 16 inside thepatient 12 in a direction perpendicular to the patient's spine.

In practice, once the device is implanted, it may shift to anotherorientation. Thus, as a practical matter it may be difficult toprecisely control the orientation of the medical implant 10. It istherefore desirable that the overall operation of the communicationsystem be relatively independent of the implant's orientation.

Although the implant 10 may shift its orientation after surgery, one cancompensate for any such shift. For example, once the incision hashealed, it is possible to determine the orientation of the implant 10.This can be achieved, for example, by X-ray inspection, or by rotatingthe receiving antenna to identify a radiation maximum. If the implant 10is sufficiently close to the skin, the orientation can be determined byfeeling the implant 10 through the skin. In either case, one can thendetermine an optimal orientation of the implant 10 relative to the basestation 14 for establishing communication with the base station 14.Information representative of this optimal orientation can then be madeavailable for the patient's use in guiding his activities, or foroptimally arranging a patient's furnishings, such as the bed and thebase station 14, to maximize likelihood of establishing and maintainingsuch communication while the patient is asleep.

FIG. 3 is a block diagram of the medical implant 10 showing atransceiver 22 having MICS circuitry 24 for communication in the MICSband, and wake-up circuitry 26 for providing a wake up signal to theMICS circuitry 24. Both the MICS circuitry 24 and the wake up circuitry26 are in communication, through matching circuits 27, 29A, 29B, with adual band antenna 28 as described in more detail in connection withFIGS. 4 and 5. A controller 30 provides control over both the MICScircuitry 24 and the wake-up circuitry 26. Implant circuitry 32 controlsthe functions of an implant device 33. A suitable transceiver 22 is theZL70101 manufactured by Zarlink Semiconductor of Ottawa, Ontario, whichis in widespread commercial use in the U.S. and other countries.

The base station 14 and transceiver 22 communicate through two frequencybands: a lower frequency band, such as the MICS band, which extends from402-405 MHz, and a higher frequency band having frequencies on the orderof 2.45 GHz. The MICS band is used primarily for data communicationbetween the transceiver 22 and the base station 14, whereas the higherfrequency band is used to provide a wake-up signal to the transceiver22, but it is not necessary that the transceiver 22 transmit back to thebase station 14 at the 2.45 GHz frequency.

FIG. 4 summarizes a procedure used by the base station 14 to establishcommunication with a medical implant 10. The base station 14 transmits awake up signal at 2.45 GHz (step 34) and then listens for a response ona MICS frequency (step 36). This wake up signal includes informationidentifying the particular MICS frequencies to be used. If no responseis forthcoming, the base station 14 retransmits the wake up signal (step34). If the base station 14 detects a response from the implant, it thenestablishes communication in the MICS band with the implant 10 (step38).

Meanwhile, the transceiver 22 on the implant 10 carries out a proceduresuch as that shown in FIG. 5.

According to FIG. 5, the wake-up circuitry 26 of the implant'stransceiver 22 periodically listens for a wake up signal at 2.45 GHz(step 40). If no signal is detected (step 42), the controller 30instructs the wake up circuitry 26 to wait for some pre-selectedinterval (step 44) and repeats this process (step 40). Otherwise, if thetransceiver 22 detects a wake up signal (step 42), it sends a signal towake up the MICS circuitry 24 (step 46) which then establishescommunication with the base station 14 (step 48). In one embodiment, thewaiting time is selected to be approximately one minute. In anotherembodiment, the controller 30 causes the wake-up circuitry 26 to listenfor the base station 14 at a particular time. In yet another embodiment,the controller 30 causes the wake-up circuitry 26 to listen for the basestation 14 at variable time intervals.

The communication protocol described in FIGS. 4 and 5 is particularlyadvantageous because the 2.45 GHz signal can be repeatedly broadcast bythe base station 14 at relatively high power, and because the wake-upcircuitry 26 on the medical implant does not have to consume power bytransmitting. Moreover, there is no need to power up the MICS circuitry24 unless MICS communication is actually required. In addition, sincethe wake-up signal identifies the portion of the MICS band to be used,there is no need for the MICS circuitry 24 to consume energy scanningacross the MICS band to search for a signal.

A difficulty that arises when attempting to communicate with animplanted transceiver 22 is that the tissues that make up the human bodygenerally have complex permittivity. As is well-known in the art, theimaginary term of a complex permittivity results in evanescent waves.Evanescent waves are essentially waves that die away, or decay, withdistance from their sources. Such waves cannot be used to carry dataover any meaningful distance since they themselves cannot travel anymeaningful distance.

Conventional antennas used in medical implants are omnidirectional.However, even though such antennas are omnidirectional, the systemformed by the union of the antenna and the human body does not radiateomnidirectionally in the space.

FIG. 6 shows a prior art implant 50 located near the ventral surface 52(i.e., the belly or stomach) of a patient 12. The implant 10 uses aconventional antenna that radiates omnidirectionally in a transverseplane. However, the power that actually leaves the patient's body in aparticular direction depends on the path length that the wave musttraverse within the body, and the permittivities that the waveencounters before reaching free space. In particular, a wave 54traveling in a ventral direction experiences little attenuation becausethe path length before reaching free space is relatively short. Incontrast, a wave 56 traveling in the dorsal direction (i.e., toward thespine or back) travels much further within the body, and thereforeexperiences more significant attenuation.

In contrast to the omnidirectional antenna shown in FIG. 6, an antenna28 as shown in FIG. 7, and as disclosed herein, provides a beam core 58directed radially outward, away from the patient's core, and a beamperiphery 60 directed to cause energy to enter the patient's peripherallayer 62. As used herein, “peripheral layer” refers to the outermostlayers of the body. Accordingly, “peripheral layer” may include thedermal layer and other tissues found in the outermost layers, such assubcutaneous fats, as well as the integumentary layer. The peripherallayer 62 has a permittivity that differs from both the free spacepermittivity and from the permittivity of the interior 64 of thepatient's body. As such, it functions in the manner of a leakywaveguide.

While not wishing to be bound by any particular physical mechanism, theantenna 28 is believed to launch an electromagnetic wave within theperipheral layer 62. Since the wave propagates in the peripheral layer62, it will be referred to herein as an “endoperipheral wave.” As itpropagates, the endoperipheral wave encounters two discontinuities inpermittivity that define the inner and outer boundaries of theperipheral layer 62. When the endoperipheral wave is incident on theouter boundary, a portion of its energy leaks across the boundary andpropagates in free space. The remaining portion is reflected back andcontinues to propagate endoperipherally.

The net effect of the foregoing arrangement as shown in FIG. 7 is areduction in the dramatically different path lengths shown in FIG. 6. Asa result, the combination of the antenna 28 and the human body shown inFIG. 7 radiates in a more omnidirectional manner than the combinationshown in FIG. 6, as shown in FIGS. 22 and 23.

The conventional antenna shown in FIG. 6 is an omnidirectional antenna.One might have expected that such an omnidirectional antenna in amedical implant 10 would provide an omnidirectional radiation pattern.But this is not the case.

In contrast, the antenna 28 shown in FIG. 7 is not an omnidirectionalantenna; it is a directional antenna. Thus one might have expected thata directional antenna in a medical implant 10 would fail to achieve anearly omnidirectional pattern.

Contrary to conventional expectation, this is not the case. Instead, thedirectional antenna 28 interacts in an unexpected way with the patient'sanatomy so that even though the antenna 28 itself is directional, thesynergy between the directional antenna 28 and the wave propagationproperties of the patient's anatomy results in a nearly omnidirectionalradiation pattern for the overall system formed by the antenna 28 andthe patient 12, as shown in FIGS. 22 and 23.

FIG. 8 shows a transverse cross section of a medical implant 10 havingan antenna 28 for launching electromagnetic waves in the mannerdescribed above. The medical implant 10 has a housing 15 on which isdisposed a bottom ground plane 66 separated from a top ground plane 68by a conducting connector 70. The top ground plane 68 and the bottomground plane 66 may also be referred to as “field stops,” “shields,” ormore generally, as a “metal surfaces,” which may or may not be planar,and which may or may not be grounded.

Within the implant housing 15 is the transceiver 22, which transmitsinformation from the body to a base station 14 outside the body orreceives information from a base station 14 outside the body. Thetransceiver 22 communicates with implant circuitry 32 that controlsoperation of an implant device 33 that interacts with the body. Oneexample of an implant device 33 is a glucose sensor as disclosed by U.S.Patent Pub. 2005/0096587 (Santini), referred to above, which is herebyincorporated by reference. Other examples of implant devices 33 includethose that perform physiological measurements and those for releasingvarious drugs. Between the top ground plane 68 and the bottom groundplane 66 is a radiating archipelago 72 comprising planar, non-wireradiating structures. A feed structure 74 disposed between the bottomground plane 66 and the radiating archipelago 72 is connected to thetransceiver 22 disposed within the housing 15. The feed structure 74,top and bottom ground planes 66, 68, radiating archipelago 72, and theconnector 70 and related structures form the antenna 28.

Transceiver 22, implant circuitry 32, and implant device 33 are sealedwithin the housing 15. Signals are passed into and out of the housing 15between transceiver 22 and antenna 28 using a feed-through structure160, which is described in more detail in connection with FIG. 16.

The top and bottom ground planes 68, 60 are separated by a dielectricmaterial, best seen in the exploded view of FIG. 9. As shown in FIG. 9,a first dielectric layer 76 separates the bottom ground plane 66 fromthe feed structure 74. A second dielectric layer 78 separates the feedstructure 74 from the radiating archipelago 72. A third dielectric layer80 separates the radiating archipelago 72 from the top ground plane 68.A bottom dielectric cover 77 isolates the bottom ground plane 66 fromcontact with any adjacent conducting media. Similarly, a top dielectriccover 79 isolates the top ground plane 68 from any adjacent conductingmedia. The top and bottom ground planes 68, 60 are connected by one ormore connectors 70, not shown in FIG. 9, that pass through the variousdielectric layers. The connectors 70 can typically be vias or metalpins. As a result, the radiating archipelago 72 is capacitively coupledto the feed structure 74 and to the top ground plane 68.

The first dielectric layer 76 is the thickest of the three. The secondand third dielectric layers 78, 80 are of approximately equal thicknessand significantly thinner than the first dielectric layer 76. The exactthicknesses of each layer depend on the properties of the dielectric andon the wavelengths to be used by the antenna 28. In one embodiment, thefirst dielectric layer 76 has a thickness of 1.27 mm and the second andthird dielectric layers 78, 80 each have a thickness of 0.1 mm.

The thicknesses of the dielectric layers 76, 78, 80 required for optimalradiation characteristics are particularly sensitive to the dielectric'spermittivity. In practice, the permittivity of a dielectric varies aboutsome nominal permittivity value from one lot or batch of material fromwhich a dielectric layer is formed to the next lot or batch from which adielectric layer is formed. Although the variations about the nominalvalue are small, and may be unimportant in many applications, in thepresent application such errors are likely to make a significantdifference in the performance of the antenna 28.

A suitable dielectric material is a biocompatible material having a highdielectric constant, which tends to reduce the overall dimensions of theantenna. In one embodiment, the dielectric material is alumina having arelative permittivity of 9.5±10% such as that supplied by DuPont undertrade designation QM44. However, other dielectrics with relativepermittivities between 9 and 10±10% (or higher) are also suitable.

In an effort to promote uniformity in manufacture, it is useful toinspect data provided by the manufacturer concerning the measuredpermittivity of a particular lot of dielectric. In one practice ofmanufacturing the antenna 28, one receives, from a supplier ofdielectric material used to form dielectric layers 76, 78, 80, ameasured value of permittivity associated with a particular lot ofdielectric material. This measured actual permittivity is oftendifferent from a nominal permittivity. This measured permittivity isthen used to determine the thickness of a layer of dielectric requiredto cause the antenna 28 to have a particular capacitance.

For example, in some manufacturing processes, particularly planarmanufacturing processes, the dielectric layers 76, 86, 80 are formed byrepeatedly painting and curing individual laminas of dielectric materialto build up a layer of dielectric 76, 86, 80 having the desiredthickness. In such cases, after having obtained the measuredpermittivity for a particular lot of dielectric, one can determine thecorrect number of laminas required to build up a dielectric layer 76,86, 80 having the desired thickness. One can then provide themanufacturing facility with instructions concerning the correct numberof laminas.

Referring to FIG. 17, a process for attaining a desired capacitanceincludes receiving, from the manufacturer, a lot or batch of particulardielectric (step 174) and a measured value of a permittivity associatedwith that lot.

The process then includes retrieving the desired capacitance (step 178)and the thickness “d” of a typical lamina of cured dielectric that wouldbe laid down by a particular manufacturing process (step 180). In atypical screen-printing process, this thickness d would correspond tothe screen thickness. A value of n, the number of laminas havingthickness d required to attain capacitance C is then obtained, either bycalculation or by use of a look-up table (step 182). The resultingvalue, n, of the number of laminas is then output (step 184) andprovided to a manufacturing facility. The manufacturing facility thenforms the requisite number of laminas to build up a layer of thickness d(step 186) and then forms a feed structure on top of the layer 76 thusformed (step 188). A similar process can be used to build up the secondlayer 78 and the third layer 80.

In many practices, the thickness of each lamina is constant. However, insome practices of the manufacturing process, the individual laminas havedifferent thicknesses. In such cases, the individual thicknesses aremade to sum to the desired thickness.

FIG. 10 shows the radiating archipelago 72 in detail. The radiatingarchipelago 72 is made up of twin MICS radiators 82, 84 that are mirrorimages of each other.

The first MICS radiator 82 includes a radiative portion 86 extendingbetween first and second reactive portions 88, 90 at opposite endsthereof. In the illustrated embodiment, the radiative portion 86 isformed by two generally parallel radiating strips 92, 94 that extendbetween the first reactive portion 88 at one end of the first MICSradiator 82 and the second reactive portion 90 at the other end of thefirst MICS radiator 82.

Similarly, the second MICS radiator 84 includes a radiative portion 96extending between first and second reactive portions 98, 100 at oppositeends thereof. In the illustrated embodiment, the radiative portion 96 isformed by two generally parallel radiating strips 102, 104 that extendbetween the first reactive portion 98 at one end of the second MICSradiator 84 and the second reactive portion 100 at the other end of thesecond MICS radiator 84. These radiative strips 102, 104 carry out afunction similar to a wire antenna. However, unlike a wire antenna,which is a three-dimensional structure, the radiative strips 92, 94,102, 104 are essentially two-dimensional structures that can easily beformed using planar processing techniques.

FIG. 11 shows a top ground plane 68 in more detail. The top ground plane68 includes an enlarged base portion 106 that is connected to the bottomground plane 66 by one or more connectors 70 through a via. Preferably,vias and connectors 70 are located away from the radiative portions 86,96 of the first and second MICS radiators 82, 84. At the opposite end ofthe top ground plane 68 is an enlarged end portion 108 connected to thebase portion 106 by an optional neck 110.

The neck 110 is disposed to shield surrounding tissues from strayelectric fields generated by the feed structure 74. The base portion 106is positioned to cover the reactive portions 88, 98 on one end of thetwin MICS radiators 82, 84. Similarly, the end portion 108 is positionedto cover the remaining two reactive portions 90, 100 on the opposite endof the twin MICS radiators 82, 84. The particular shapes of the end andbase portions 106, 108 are not critical to their overall function.

The end portion 108 of the top ground plane 68 and the two reactiveportions 90, 100 of the MICS radiators 82, 84 lie on opposite sides ofthe third dielectric layer 80. As such, they collectively define a firstcapacitor 112 between them, as shown in FIG. 8. The base portion 106 ofthe top ground plane 68 and the two remaining reactive portions 88, 98also lie on opposite sides of the third dielectric layer 80. As such,they collectively form a second capacitor 114, shown in FIG. 8. Each ofthese capacitors 112, 114 entraps electric field lines, thus suppressingthe tendency of the antenna's near field to heat or to otherwise bedissipated by the human tissue adjacent to the implant 10. Instead ofbeing lost as heat or as dielectric losses, the energy in the near fieldis available to oscillate from one end of the MICS radiators 82, 84 tothe other, preferably at a resonance frequency associated with the MICSradiators 82, 84. As a result, this energy can contribute moresignificantly to far-field radiation.

Waves that ultimately reach the far field of the antenna 28 originateprimarily from the radiative portions 86, 96. Since these radiativeportions lie underneath and on opposite sides of the neck 110 of the topground plane 68, there is little to impede wave propagation from theseportions. In embodiments that lack any neck, nothing at all impedes wavepropagation. As a result, those waves are free to propagate into the farfield of the antenna 28.

As used herein, the “far field” of an antenna, sometimes referred to asthe “radiation field,” is used in a manner consistent with the way it isused in the antenna arts. In particular, the “far field” is the regionof space that is so remote from the antenna that the electromagneticfield of the antenna, which normally includes an evanescent componentand a radiating component, consists primarily of the radiatingcomponent.

Referring back to FIG. 7, an antenna 28 as described herein provides apattern having a main lobe 58 that radiates energy in a radial directionaway from the patient 12. In addition, some of the energy stored in thenear field at the ends of the MICS radiators 82, 84 escapes through thegap between the top and bottom ground planes 66, 68. This energy ismanifested as side lobes 60 shown in FIG. 7. It is these side lobes 60that are believed to provide the energy for launching the endodermalwave.

An antenna 28 as described above has a relatively low radiationefficiency, i.e. only a small portion of energy delivered to theradiative portions 86, 96 is actually radiated. The bulk of the energyremains stored in the near field of the antenna 28 rather than beingradiated away.

In operation, the transceiver 22 provides energy through a feed point116, best seen in FIGS. 8 and 14. The energy propagates along the feedstructure 74 and couples capacitively to the radiative portions 86, 96.Once on the radiative portions 86, 96, the energy travels towards thereactive portions 90, 100. A small portion of that energy radiates fromthe relatively inefficient radiating strips 92, 94, 102, 104 of theradiative portions 86, 96. The bulk of the energy reaches the reactiveportions 90, 100 and reflects back to travel again along the radiativeportions 86, 96, where another small portion radiates away. Theremainder then proceeds to the opposite reactive portion 88, 98, whichthen reflects it back along the radiative portions 86, 96.

The reactive portions 90, 100, 88, 98 and radiative portions 86, 96 thuscooperate to cause energy to oscillate back and forth between thereactive portions 90, 100, 88, 98. At each oscillation, a small portionof that energy radiates away as it traverses the radiative portions 86,96. Thus, even if the radiation efficiency of each radiative portion 86,96 is relatively low, the minimal energy radiated with each oscillationaccumulates and eventually provides sufficient power to communicate witha base station 14 located at some distance away. For example it isbelieved that this arrangement will permit communication within the sameroom approximately five meters away.

The operation of the antenna 28 thus provides another unexpected result.Ordinarily, one would expect to increase range by increasing efficiency,i.e., by providing an antenna 28 that has high radiation resistance.This would translate into a greater fraction of energy being radiated inthe far field of the antenna 28. While this may be the desirablesolution in free space, the limited space within the human body makes itdifficult to implant a large enough antenna to have a high radiationresistance in the MICS band. However, implanting a small antenna withlow radiation resistance causes more energy to be retained in theantenna's near field. Since the antenna near field lies within humantissue, this results in dielectric losses.

To overcome the foregoing disadvantage of using an electrically smallantenna in a lossy dielectric medium, the reactive portions 90, 100, 88,98 are shielded by the top ground plane 68. The shielding constrainsnear fields from spilling out into the surrounding tissue. As a result,dielectric loss is reduced.

Instead of adopting the conventional solution, the antenna 28 describedherein is a highly inefficient antenna, i.e., one with a low radiationresistance. In such a highly inefficient antenna, only an insignificantfraction of energy provided to the antenna actually radiates into thefar field. Nevertheless, by entrapping the bulk of the energy andbleeding it into the far field a little bit at a time through relativelyinefficient radiative portions 86, 96, the antenna 28 avoids lossesarising from interaction between its near field and surrounding humantissue. This leads to the unexpected result of an inefficient antenna 28that nevertheless manages to provide long range wireless communicationbetween a medical implant 10 and a base station as much as 5 metersaway.

In operation, the antenna 28 is analogous to a laser oscillator, inwhich light oscillates between two mirrors with only a small portion ofthe light escaping through a half-silvered mirror with each oscillation.

The antenna 28 can be viewed as an RLC circuit in which the resonantfrequency, which is the reciprocal of the square root of the product ofthe effective inductance and capacitance, is within the desiredfrequency band of operation, i.e. the MICS band. The relatively smallradiation resistance, as well as the inductance, is provided by theradiative portions 86, 96. The capacitance, which dominates theillustrated configuration, is provided by the two capacitors 112, 114formed by the interaction between the reactive portions 100, 88, 90, 98of the MICS radiators 82, 84 and the base and end portions 106, 108 ofthe top ground plane 68.

In another embodiment, the RLC circuit is dominated by inductance ratherthan capacitance. In that case, the reactive portions of the MICSradiators 82, 84 are meander line structures 118, 120 such as thoseshown in FIG. 12. In this embodiment, the radiating strips 122, 124 aremade somewhat wider so that they can provide the necessary capacitanceto tune the antenna 28.

As discussed above, the transceiver 22, and hence the antenna 28,operates on two frequencies: one in the MICS band and another, in theUHF band, for carrying the wake-up signal. As used herein, “UHF” meansone of the ISM (Industrial, Scientific, Medical) bands, andspecifically, the ISM band that includes frequencies between 2.4 GHz and2.5 GHz. To accommodate the second frequency, an alternative embodimentof the radiating archipelago 72 shown in FIG. 13 features UHF radiators126, 128 tuned to resonate at 2.45 GHz, as shown in FIG. 10. Like theMICS radiators 82, 84, the UHF radiators 126, 128 are twin radiatingelements, each one being an essentially linear structure. The first UHFradiator 126 has a first reactive portion 130 and a second reactiveportion 132 connected by a radiative portion 138 extending between them.Similarly, the second UHF radiator 128 has a first reactive portion 134and a second reactive portion 136 connected by a radiative portion 140extending between them.

As used herein, the use of the term “radiative” portion is not intendedto imply that the structure can be used only for transmittingelectromagnetic waves. As is well known in the art, antennas are subjectto reciprocity. Hence, structures used for transmitting waves have thesame properties when used for receiving electromagnetic waves.

In an alternative embodiment, as shown in FIG. 19, the radiativeportions 138, 140 of the first and second UHF radiators 126, 128 areformed into radiative strips like those shown on the MICS radiators 82,84.

As shown in FIG. 11, in those embodiments that include the optional neck110, the neck 110 can include a central neck 142 and two peripheralnecks 144, 146, with each peripheral neck 144, 146 connecting thecentral neck 142 to one of the end portion 108 and base portion 106. Theperipheral necks 144, 146 are both wider than the central neck 142, butnot so wide as to interfere with propagation of waves escaping from theradiative portions 82, 84 of the MICS radiators. However, the peripheralneck portions 144, 146 are nevertheless wide enough to form a pair ofcapacitors with the reactive portions 130, 132, 134, 136 of the twin UHFradiators 126, 128.

It is thus apparent that the operation of the UHF radiators 126, 128 isidentical to that of the MICS radiators 82, 84, with the two peripheralneck portions 144, 146 of the top ground plane 68 playing the roles withrespect to the UHF radiators 126, 128 that the end and base portions106, 108 of the top ground plane 68 played with respect to the MICSradiators 82, 84.

In one embodiment, the top ground plane 68 has: (1) a central neck 142having a length of 4 mm and a width of 1.1 mm; and (2) a pair of 5.1 mmwide peripheral necks 144, 146 having lengths of 6.85 mm long and 10.45mm respectively. The base portion 106 of the top ground plane 68 is asemicircular region having a radius of 9 mm. The end portion 108 is asemicircular region having a radius of 9 mm contiguous with arectangular region extending 4 mm towards the base portion 106 and 17.8mm along a direction perpendicular to the major axis 16 of the implant10.

A bottom ground plane 66 corresponding to the above top ground plane 68is a rectangular region extending 25.2 mm along the major axis 16 and18.82 mm perpendicular to the major axis 16. Each 18.82 mm side of therectangular region is contiguous with a semicircular region having aradius of approximately 9.4 mm.

Referring now to FIG. 14, the feed structure 74 is an axial transmissionline 143 extending from the feed point 116 along the major axis 16. Atthe distal tip of the axial transmission line 143 is a distal load 145formed by two short sections 147, 149 of transmission line extendingperpendicularly from the axial transmission line 143 in oppositedirections underneath the reactive portions 88, 98 of the MICS radiators82, 84. At an intermediate point of the transmission line, under the UHFradiators 126, 128, is an intermediate load 150 formed by an additionalpair of transmission line sections 152, 154 extending perpendicularlyfrom the axial transmission lines 143.

A distal section 148 of the axial transmission line 143 extends betweenthe distal load 145 and the intermediate load 150. A proximal section141 of the axial transmission line 143 extends between the intermediateload 150 and the feed point 116. A suitable diplexing feed structure 74for the radiating archipelago 72 whose numerical dimensions have beenprovided features a distal section 148 having a length of approximately16.75 mm, and a proximal section 141 having a length of approximately11.24 mm. The axial transmission line 143, the intermediate load 150 andthe distal load 145 cooperate to form a diplexing feed structure 74, ordiplexer.

The use of a diplexing feed structure 74 makes it possible to use asingle coaxial cable instead of a pair of coaxial cables to provideenergy to the feed structure 74. This is particularly advantageous wherethe device is one in which space is at a premium, for example in amedical implant 10.

However, the use of a diplexing feed structure 74 is by no meansmandatory for operation of the antenna 28. The antenna 28 can also beexcited by two separate coaxial cables or other transmission linescarrying signals in two different frequency bands.

A suitable diplexing feed structure 74 for the radiating archipelago 72whose numerical dimensions have been provided features an axialtransmission line 143 extending 31.5 mm between the feedpoint 116 andthe distal load 145. A pair of 1 mm wide transmission line sections 152,154 extending 3 mm on either side of the axial transmission line 142provides the intermediate load 150. A pair of transmission line sections147, 152 2.5 mm wide extending 1.35 mm on either side of the axialtransmission line 143 provides the distal load 144.

In another embodiment of the feed structure 74, shown in FIG. 20, anaxial transmission line 200 extends along the axis 16 of the implant 10between the feed point 116 and a distal load 202 formed by a pair oftransmission line stubs 204, 206 extending perpendicular to the axialtransmission line 200 in opposite directions. The distal load 202 isdisposed to capacitively couple with the MICS radiators 82, 84. Alsoextending from the feed point 116 are a pair of transmission lines 208parallel to and offset from the axial transmission line 200. Each of thepair of transmission lines 208 ends at an intermediate load 210 thatcapacitively couples to one of the UHF radiators 134, 136.

In some embodiments, as shown in FIG. 21, an additional load is providedby introducing a tuning stub formed by proximal and distal right-anglebends in the axial transmission line 143. These two right-angle bendsare connected by a connecting section of transmission line parallel tobut offset from the axial transmission line 143. The connecting sectionin one embodiment is 1 mm long and offset by 1.5 mm from the axialtransmission line 143. The proximal right-angle bend is approximately23.7 mm from the feedpoint 116, and the distal right-angle bend is anadditional 1 mm further from the feedpoint 116.

In operation, with reference for example to FIG. 14, a wave formed bythe superposition of an MICS component and a UHF component originates atthe feed point 116 and propagates along the axial transmission line 142.The impedance as seen by the UHF component is such that the distal load145 appears as an open or short circuit, whereas the intermediate load150 is matched to the UHF radiators 134, 136. As a result, the UHFcomponent is effectively coupled into the UHF radiators 134, 136 andrejected by the MICS radiators 82, 84. Conversely, the impedance as seenby the MICS component is such that the intermediate load 150 appears asan open or short circuit and the distal load 145 is matched to the MICSradiators 82, 84. As a result, the MICS component is effectively coupledto the MICS radiators 82, 84 and rejected by the UHF radiators 134, 136.

In some embodiments, the impedances are neither those of short circuitsnor of open circuits. In these embodiments, the impedances include afinite and non-zero imaginary (i.e., reactive) component. Typically, thereactive component is capacitive; however, for certain configurationsthe reactive component is inductive.

As shown in the exploded view of FIG. 9, the feed structure 74 isdisposed in its own layer between the radiating archipelago 72 and thebottom ground plane 66. A disadvantage of this configuration is that itrequires an additional metal layer, and thereby complicatesmanufacturing. In another embodiment, the feed structure 74 and theradiating archipelago 72 are on the same dielectric layer. In thisembodiment, the feed structure 74 is directly connected to selectedportions of the radiating archipelago 72 rather than being capacitivelycoupled to those portions. Such a configuration is less sensitive toerrors in manufacture since there is no longer a need to rely oncapacitive coupling between the feed structure 74 and the radiatingarchipelago 72.

Placement of the feed structure 74 and radiating archipelago 72 on thesame layer does not, however, eliminate the possibility of a capacitivecoupling between the feed structure 74 and the radiating archipelago 72.For example, FIG. 18 shows UHF radiators 126, 128 capacitively coupledto the feed point 116 using planar capacitors 127 formed byinterdigitating conductive traces 129 connected to the UHF radiators126, 128 with conductive traces 131 connected to the feed point 116.

In one embodiment, the radiating archipelago 72 extends 15.3 mm from anoutermost edge of one outer radiating strip 92 of one MICS radiator 82to an outermost edge of an outer radiating strip 102 of the other MICSradiator 84, and 36.9 mm from the tip of one reactive portion 88 to theother reactive portion 90. Each radiating strip is about 1.5 mm wide and21.2 mm long. Each pair of radiating strips 102, 104 is separated by agap of approximately 0.44 mm. Each UHF radiator 126, 128 has a radiativeportion 138 approximately 3.9 mm long and 1 mm wide. Each UHF radiator126 has reactive portions 130, 134 at each end, with the reactiveportions 130, 134 being formed by a metal strip approximately 2.8 mmlong and 1 mm wide extending in a direction perpendicular to theradiative portion 138.

Thus, in the MICS band, where the free-space wavelengths are on theorder of 0.75 meters, the overall electrical length of the MICSradiators 82, 84 amounts to an insignificant fraction of a wavelength.

FIG. 15A shows one embodiment of a matching circuit 27 to match thetransceiver 22 to the antenna 28. In the illustrated embodiment, thetransceiver 22 has an input impedance in the UHF band of 2 kilo-ohms,and an input impedance in the MICS band of 500 ohms when transmittingand 20 kilo-ohms when receiving. The antenna 28 has a 50 ohm inputimpedance. A coaxial cable 156 with characteristic impedance of 50 ohmsconnects the antenna 28 to the matching circuit 27.

The illustrated matching circuit 27 features two paths, one for eachband. A first path connects the transceiver 22 directly to the antenna28 by way of coupling capacitor C₁. A second path uses a couplingcapacitor C₂ and coupling inductor L₂ to connect the transceiver 22 tothe antenna 28 by way of an LC circuit 158 made up of inductor L₁ inparallel with capacitor C₃. This second path is tuned by a variableshunt capacitor CV.

In one embodiment, coupling capacitor C₁ has a capacitance ofapproximately 0.5 picofarads, coupling capacitor C₂ has a capacitance ofbetween about 0.5 and 5 picofarads, coupling inductor L₂ has a valuebetween 15 nH and 50 nH, and preferably at or near 22 nH, and thevariable capacitance C_(v) has a capacitance ranging from 5 to 60picofarads. The LC circuit in this embodiment includes a capacitance C₃of approximately 1 picofarad and an inductance L₁ of approximately 3nanohenries.

In another embodiment, components within the chip that houses thetransceiver 22 are incorporated into the matching circuit 27. Like thematching circuit of FIG. 15A, the matching circuit 27 of FIG. 15Bfeatures two paths, one for each band. A first path connects the 2.45GHz receiving port (RX_UHF) of the transceiver 22 directly to theantenna 28 using a coupling capacitance C₁ in series with a high-passfilter 226 formed by a capacitance C₂ and inductance L₁. A second pathuses a stop-band filter 224 formed by capacitor C₃ in parallel withinductor L₂ in series with a pi-matching network 228. The pi-matchingnetwork 228 is formed by an inductor L₃ having one terminal connected toground by a capacitor C₅ internal to the chip housing the transceiver 22and another terminal connected to ground by a DC coupling capacitance C₄in series with parallel capacitors C_(TX) and C_(RX), both of which arealso internal to the transceiver 22. The capacitor C_(TX), atransmission port TX-RF of the transceiver 22 by an amplifier TX and thecapacitor C_(RX) is coupled to a receiving port RX-RF of the transceiver22 by an amplifier RX.

A feed-through 160, as shown in FIG. 16, provides a connection betweenthe coaxial cable 156 and the antenna 28. This allows the circuitcomponents to be sealed within housing 15 and the antenna 28 to beplaced on an external surface of the housing 15. Thus, the RFtransparent cover 20 can be made of simple construction to keep theantenna 28 clear of body fluids. The location of the feed-through 160relative to the matching circuit 27 is shown schematically in FIGS. 15Aand 15 B.

The feed-through 160 includes an annulus 162 having an outer rim 164 andan inner rim 166. The annulus 162 is sized so that the outer rim 164engages the sides of a hole in the bottom ground plane 66 at the feedpoint 116, as shown in FIG. 8. A dielectric plug 168 shown in FIG. 16fills the space defined by the inner rim 166. First and secondconductors 170, 172 extend through the dielectric plug 168. The firstconductor 170 extends to the feed structure 74 while the secondconductor 172 contacts the bottom ground plane 66. In this way, thefeed-through 160 provides electrical contact between the antenna 28 andthe matching circuit 27.

FIGS. 22 and 23 show simulated three-dimensional radiation patterns forthe antenna at 403.5 MHz (in the MICS band) and at 2.45 GHzrespectively. The patterns were computed using the finite-element methodas implemented by HFSS software provided by Ansoft Corporation ofPittsburgh, Pa.

In both figures, the y-axis corresponds to the major axis 16 of thehousing 15, the z-axis corresponds to the direction away from thepatient's body, and the −z direction corresponds to a direction into thepatient's body. As is apparent from the figures, at each band thereexist nulls in the direction of the major axis and an approximatelyomnidirectional pattern in a plane transverse to the major axis 16 ofthe housing 15. As is also apparent from the figures, there exists asmall amount of loss in the −z direction that arises as a result ofdielectric and conductive losses in the layer amount of tissue that istraversed in that direction.

FIGS. 24 and 25 each show three planar slices through thethree-dimensional radiation patterns of FIGS. 22 and 23 respectively,one corresponding to a slice containing the yz plane (φ=900), anothercontaining the xz plane (φ=0°), and a third containing a plane midwaybetween the xz and yz planes (φ=45°).

In an effort to confirm that an antenna as disclosed herein wouldfunction as predicted within the MICS band, a link budget was prepared.A constraint imposed on the link budget was that for any directionwithin 40 degrees of the antenna beam's maximum, the power available atthe base station 14 would be at least −90.1 dBm when the transmittedpower was −3 dBm. The link budget assumed a −1 dBm loss in the matchingcircuit and a −2.7 dBm loss for transmission in a direction of fortydegrees off-axis. Transmission across five meters was assumed to resultin another −39 dBm loss. A fading margin of −5 dB was assumed in thelink budget to account for multipath interference between the antennaand the base station. At the base station 14, the receiving antenna wasassumed to have a 0 dB gain and a matching circuit loss of −1 dB.

An antenna as described herein was implanted beneath a layer of fat inpig meat. An antenna gain in the on-axis direction was then measured atfrequencies between 360 MHz and 440 MHz in an anechoic chamber using afirst antenna under approximately one inch of fat, and using the firstantenna and a second antenna under approximately half an inch of fat.The resulting on-axis gains as a function of frequency are shown in FIG.26.

According to FIG. 26, the on-axis gain at 403.4 MHz was approximately−24 to −25 dB. When this gain was used in the link budget, the powerthat entered the transceiver 22 following a −3 dBm transmission in adirection 40 degrees off-axis across five meters of free space was foundto be adequate for reliable communication.

A similar experiment was carried out for an antenna in the UHF band,specifically at 2.45 GHz. In this experiment, the link budget assumed atransmission of 21 dBm from a base station 14. A matching circuit lossof −1 dB and antenna gain of 0 dB were assumed at the base station 14.Over a five meter free space propagation distance, a loss of −54 dB wasassumed, with an additional −2.5 dB loss due to multipath interference.A 0 dB loss was assumed for a matching circuit at the transceiver 22.

An antenna as described herein was implanted beneath a half inch layerof fat in pig meat. An on-axis antenna gain was then determined in ananechoic chamber by sweeping across a frequency band extending between2.25 GHz and 2.60 GHz using a first antenna under approximately one inchof fat, and using the first antenna and a second antenna underapproximately half an inch of fat. The resulting on-axis gains as afunction of frequency are shown in FIG. 27. As is apparent from FIG. 27,the on-axis antenna gain at 2.45 GHz was between approximately −16 dBand −18 dB. When these values of receiving antenna gain were assumed inthe link budget, the resulting power available at the transceiver 22following a 21 dBm transmission from a base station 14 in a directionforty degrees off-axis across five meters of free space was found to besufficient to reliably detect a wake-up signal at the transceiver 22.

LIST OF REFERENCE NUMERALS

-   -   10 medical implant    -   12 patient    -   14 base station    -   15 housing of implant    -   16 major axis of implant    -   18 locking ring on housing    -   20 RF-transparent cover    -   22 transceiver    -   24 MICS circuitry    -   26 wake-up circuitry    -   27 matching circuit    -   28 antenna    -   29A, 29B matching circuits (internal to transceiver)    -   30 controller    -   32 implant circuitry    -   33 implant device    -   50 medical implant    -   52 ventral surface of patient    -   54 wave traveling ventrally    -   56 wave traveling dorsally    -   58 main lobe of antenna    -   60 side lobe of antenna    -   62 dermal layer    -   64 interior of patient    -   66 bottom ground plane    -   68 top ground plane    -   70 connector between top and bottom ground planes    -   72 radiating archipelago    -   74 feed structure    -   76, 78, 80 dielectric layers    -   77, 79 dielectric covers    -   82, 84 MICS radiators    -   86, 96 radiative portions of MICS radiators    -   88, 90, 98, 100 reactive portion of MICS radiators    -   92, 94, 102, 104 radiative strips of MICS radiators    -   106 base portion of top ground plane    -   108 end portion of top ground plane    -   110 neck of top ground plane    -   112 first capacitor    -   114 second capacitor    -   116 feed point    -   118, 120 meanderline structures    -   122, 124 radiative strips of meanderline antenna    -   126, 128 UHF radiators    -   127 planar capacitor    -   129, 131 conductive traces of planar capacitor    -   130, 132, 134, 136 reactive portions of UHF radiators    -   138, 140 radiative portions of UHF radiators    -   141 proximal section of axial transmission line    -   142 central neck portion of top shield    -   143 axial transmission line    -   144, 146 peripheral neck portions of top shield    -   145 distal load    -   147, 149 distal load transmission line stubs    -   148 distal section of axial transmission line    -   150 intermediate load    -   152, 154 intermediate load transmission line stubs    -   156 coaxial cable    -   158 LC circuit    -   160 feed through    -   162 frame of feed through    -   164 outer rim of frame    -   166 inner rim of frame    -   168 dielectric plug    -   170 first conductor    -   172 second conductor    -   200 axial transmission line    -   202 distal load    -   204, 206 transmission line stubs    -   208 transmission lines    -   210 intermediate load    -   215 axial transmission line    -   218 distal load    -   220 intermediate load    -   222 transmission    -   224 stop-band filter    -   226 high-pass filter    -   228 pi-matching network

1. An apparatus for providing transdermal wireless communication, saidapparatus comprising: medical implant circuitry; a transceiver coupledto the medical implant circuitry; a first metal surface, the first metalsurface having an end portion and a base portion; a second metal surfaceparallel to the first metal surface and connected to the first metalsurface by a conductor, the second metal surface being separated fromthe first metal surface by a dielectric layer; a first radiating elementtuned to a first frequency and disposed within the dielectric layerbetween the first metal surface and second metal surface, the firstradiating element having a first reactive portion at a first endthereof, a second reactive portion at a second end thereof, and a firstradiating strip extending between the first reactive portion and thesecond reactive portion; and a feed structure in electricalcommunication with the transceiver and the first radiating strip.
 2. Theapparatus of claim 1, wherein the first reactive portion comprises afirst capacitive structure, and the second reactive portion comprises asecond capacitive structure.
 3. The apparatus of claim 1, wherein thefirst reactive portion comprises a first conductive planar portionhaving a dimension in excess of a width of the first radiating strip,and wherein the second reactive portion comprises a second conductiveplanar portion having a dimension in excess of the width of the firstradiating strip.
 4. The apparatus of claim 1, wherein the first reactiveportion comprises an inductive structure.
 5. The apparatus of claim 1,wherein the first reactive portion comprises a first conducting stripdisposed to follow a first serpentine path, and the second reactiveportion comprises a second conducting strip disposed to follow a secondserpentine path.
 6. The apparatus of claim 1, wherein the first reactiveportion of the first radiating element comprises an inductive structureand the second reactive portion comprises a capacitive structure.
 7. Theapparatus of claim 1, wherein the feed structure is separated from thefirst radiating strip by a dielectric.
 8. The apparatus of claim 1,wherein the feed structure is capacitively coupled to the firstradiating strip.
 9. The apparatus of claim 1, further comprising asecond radiating element tuned to a second frequency, the secondradiating element being disposed between the first reactive portion ofthe first radiating element and the second reactive portion of the firstradiating element.
 10. The apparatus of claim 9, wherein the feedstructure provides a signal of the first frequency and a signal of thesecond frequency to both the first radiating element and the secondradiating element.
 11. The apparatus of claim 9, wherein the feedstructure is capacitively coupled to both the first radiating elementand the second radiating element.
 12. The apparatus of claim 9, whereinthe second radiating element is tuned to a frequency betweenapproximately 2 GHz and 2.5 GHz.
 13. The apparatus of claim 1, wherein aplanar surface forms the second metal surface.
 14. The apparatus ofclaim 13, wherein the planar surface comprises a surface of a housing.15. The apparatus of claim 1, wherein the second metal surface comprisesa planar surface of a housing.
 16. The apparatus of claim 1, wherein theend portion of the first metal surface is disposed over the firstreactive portion of the first radiating element and wherein the baseportion of the first metal surface is disposed over the second reactiveportion of the first radiating element.
 17. The apparatus of claim 1,wherein the first radiating element defines a region at which anelectric field supported by the first radiating element has its maximumamplitude, and wherein at least one of the end portion and base portionis disposed over the defined region.
 18. The apparatus of claim 1,wherein at least one of the end portion and base portion is disposedover a region at which a gradient vector of electric field amplitudereverses direction.
 19. The apparatus of claim 1, wherein at least oneof the end portion and base portion is disposed to intercept electricfield lines in a region at which an electric field supported by thefirst radiating element reaches its maximum amplitude.
 20. The apparatusof claim 1, wherein at least one of the end portion and base portion isdisposed to block a selected portion of an electric field supported bythe first radiating element.
 21. The apparatus of claim 20, wherein theselected portion of the electric field is a portion having an amplitudein excess of a threshold.
 22. The apparatus of claim 20, wherein theselected portion of the electric field is a portion having a gradientvector that reverses direction.
 23. The apparatus of claim 1, whereinthe first radiating element is tuned to a first frequency within theMICS band.
 24. The apparatus of claim 1, wherein the first radiatingelement is tuned to a first frequency between 400 MHz and 405 MHz. 25.The apparatus of claim 1, further comprising a neck extending betweenthe end portion and the base portion.
 26. The apparatus of claim 1,wherein at least one of the first metal surface and the second metalsurface comprises a planar surface.
 27. The apparatus of claim 1, wherethe first and second metal surfaces comprise grounded surfaces.
 28. Theapparatus of claim 1, where the first and second metal surfaces compriseground planes.
 29. An apparatus for providing transdermal wirelesscommunication in a selected direction, said apparatus comprising:medical implant circuitry; a transceiver coupled to the medical implantcircuitry; a first metal surface disposed in a plane perpendicular tothe selected direction, the first metal surface having an end portion,and a base portion, and a first planar radiating element tuned to afirst frequency and disposed on a dielectric layer above the first metalsurface, the first planar radiating element having a first reactiveportion at a first end thereof, a second reactive portion at a secondend thereof, and a first planar radiating strip extending between thefirst reactive portion and the second reactive portion; and a feedstructure in electrical communication with the transceiver and the firstplanar radiating strip for providing a carrier signal to the firstplanar radiating strip.
 30. The apparatus of claim 29, furthercomprising a neck extending between the end portion and the baseportion.